In a large dutch oven, add potatoes and enough cold water to cover. Bring to a boil, reduce heat and simmer until tender - about 15-20 minutes, drain thoroughly. Return potatoes to pan. Add 3/4 cup fontina cheese, 1/2 cup Gruyère cheese, butter, salt, and pepper to. Using a potato masher, mix until well combined. Pour in the warm milk and continue mashing until it is the consistency that you prefer.

Scoop mixture into a 9 x 13" baking dish coated with nonstick spray. Scatter the top evenly with the remaining fontina and Gruyère cheese. Cover with foil and bake for 20 minutes. Remove the foil and preheat the broiler. Broil the mixture until the cheese is bubbly and has started to brown.